This invention relates to an active matrix display device having a row and column array of display elements comprising spaced electrodes with electro-optical material therebetween and connected between sets of row and column address conductors, and in which the display elements of a row are each connected to a respective address conductor of one set via two parallel switch means which each comprise at least two, bidirectional, non-linear devices connected in series.
A display device of the above kind may be used for datagraphic or video, e.g. T.V., display purposes.
An active matrix liquid crystal display device in which each display element is connected in series with a switch means between row and column address conductors carried on opposed supporting plates and in which each switch means comprises two bidirectional thin film diodes (TFDs) is disclosed in the paper by Hirai et al entitled "Flicker-Free LCD with Bridge Structure SiNx TFD" in SID 90 Digest, pages 522 to 525. By using two TFDs the switch means is given a substantially symmetrical current/voltage characteristic when driven using voltages of one polarity and voltages of the opposite polarity.
It is known also in active matrix display devices generally to connect a plurality of switch means in parallel between a display element electrode and an associated address conductor for redundancy purposes. An example of such a display device is described in EP-A-0460747. In this display device the display elements, comprising liquid crystal elements, are each defined by a pair of display element electrodes carried on respective glass supporting plates between which liquid crystal material is disposed. The plates carry respectively sets of row and column address conductors and individual display elements are provided at the region of the intersections of the crossing row and column address conductors. One display element electrode of each display element in a row is connected to a respective address conductor on the one plate via a switch means comprising two bidirectional thin film diodes (TFDs) in the form of MIM elements connected in series which are also carried on that plate. The two series connected MIM elements associated with a display element act as a switch means having a threshold characteristic which, in response to a predetermined voltage applied between the row and column address conductors associated with that display element, is turned on so as to transfer a charge to the display element thereby causing a desired display effect. In an embodiment of a display device disclosed in this specification, each display element electrode is connected to the address conductor on the one plate via a plurality of such switch means connected in parallel between the display element and the address conductor.
The use of the two series-connected TFDs, such as MIMs, rather than a single TFD as is more common, is important particularly for datagraphic displays. Ageing effects in TFDs cause drift in their operational behaviour (I-V characteristic) and in the case of a single TFD being used the extent of such drift may differ for individual display elements if their associated TFDs do not on average receive the same drive voltages, and thus suffer the same stress, over a period of time in operation. By using two TFDs, in series, the amount of shift is reduced.
In fabricating the display devices, defects can occasionally occur in the TFDs. For a large area display device having a few hundred thousand TFDs the probability of one, or more, TFDs being found to be faulty is not insignificant. Most often the fault is likely to be a short so that the defective TFD is permanently short-circuited between its terminals. However, faults leading to a permanently open circuit between the TFD's terminals can also occur. By providing two (or more) switch means in parallel between a display element and its address conductor, problems caused by defective devices can be overcome. In the case for example of a TFD in one switch means being open circuit then the display element is driven simply through the other switch means. In the case of a TFD being short-circuit, the relevant faulty switch means can be permanently disconnected in known manner from the display element's circuit by, for example, laser scribing, leaving the other switch means then to operate alone in driving the display element. However, before such disconnection can be effected, it is necessary to identify the particular TFD which is defective, or at least the switch means containing the defective TFD, and this can prove difficult and time consuming. One technique, for example, involves the visual inspection of individual circuit elements under a microscope.